How South Korea’s legendary female free divers evolved for a life underwater

An island 50 miles (80 kilometers) off the southern tip of the Korean Peninsula is home to a unique and celebrated community of women: the Haenyeo. These women dive year-round off Jeju Island, collecting sea urchin, abalone and other seafood from the ocean floor, descending as much as 60 feet (18 meters) beneath the surface multiple times over the course of four to five hours each day. They dive throughout pregnancy and well into old age, without the help of any breathing equipment — just a wet suit. “For thousands of years, we think, they’ve been doing this incredible, matrilineal thing, where they learn from the mother how to dive at a very young age. They go out in these collectives, and that’s what they do. They dive,” said Melissa Ann Ilardo, a geneticist and assistant professor in biomedical informatics at the University of Utah. “They’re spending really an extraordinary percentage of their time underwater.” Ilardo, along with colleagues in South Korea, Denmark and the United States, wanted to understand how the women manage this incredible physical feat. Specifically, the researchers wondered whether the divers have unique DNA that allows them to go without oxygen for so long or if that ability is the result of a lifetime of training — or a combination of the two. The findings of their investigation, published in the scientific journal Cell Reports on May 2, uncovered unique genetic differences the Haenyeo have evolved to cope with the physiological stress of free diving. It’s a discovery that could one day lead to better treatments for blood pressure disorders, researchers say. “It’s a beautiful island, like sometimes they call it the Hawaii of Korea. There’s this coastline everywhere that’s rich with great resources, so you can imagine any population living in a place like that of course you would want to take advantage of it,” Ilardo said. Diving has been part of Jeju culture for many years. It’s unclear at what point it became a women-only activity, but theories include a tax on male divers or a shortage of men, Ilardo said. Still, diving is so integral to Jeju’s population that the shortening of words characteristic of the Jeju language is attributed to the need for divers to communicate quickly, according to the new study. However, the practice is dying out. Young women are no longer continuing this matrilineal tradition; the current group of Haenyeo divers, with an average age of around 70 years, may represent the last generation, the researchers noted in the study. Diving into DNA For their research, Ilardo and her colleagues recruited 30 Haenyeo divers, 30 non-diving women from Jeju and 31 women from the South Korean mainland. The average age of the participants was 65. The researchers compared participants’ heart rates, blood pressure and spleen sizes and sequenced their genomes — a detailed genetic blueprint — from blood samples. The study’s biggest challenge was safely replicating the physical stress of being underwater for relatively long periods for participants with no diving experience, Ilardo noted. The researchers solved this problem by conducting simulated dives, during which participants held their breath while submerging their faces in cold water. “We would have loved to collect these measurements from everyone in the open ocean, but obviously you can’t ask 65-(year-old), 67-year-old women who have never dived before in their lives to hop in the water and hold their breath and dive,” Ilardo said. “Fortunately, if you hold your breath and put your face in a bowl full of cold water, your body responds as if you’re diving. And that’s because the nerve that stimulates the mammalian dive reflex goes through your face,” she said. When you feel the cold water combined with the breath hold, “your body says, ‘oh I’m diving’: So your heart rate slows down, your blood pressure increases and your spleen contracts,” she added. The team’s analysis revealed that the participants from Jeju — both divers and non-divers — were more than four times more likely than mainland Koreans to have a genetic variant associated with lower blood pressure. “Your blood pressure increases as you dive. Their (Jeju residents’) blood pressure increases less,” Ilardo explained. The researchers believe the trait may possibly have evolved to keep unborn children safe because the Haenyeo dive throughout pregnancy, when high blood pressure can be dangerous. The team also found that the Jeju participants were more likely to have a genetic variation previous research has linked to cold and pain tolerance. However, the researchers did not measure the participants’ ability to withstand low temperatures, so they can’t say for sure whether the variant may be important for the Haenyeo’s ability to dive year-round. “Throughout winter they’re diving when it’s snowing, and up until the 1980s, they were doing that in cotton with no protection at all. There’s a lot more that we need to explore and find the answers to,” Ilardo said. The Haenyeo’s diving prowess didn’t come down to genetics alone. The study also found that the female divers had a slower heart rate than non-divers during the tests — a factor that would help them to conserve oxygen during a dive. “It was quite dramatic. Actually, their heart rate dropped about 50% more over the course of the dive than the control (participants). We know that it’s because of training, because it’s something that we only saw in the Haenyeo,” Ilardo said. Ilardo’s previous work involving free diving communities known as the Bajau in Sulawesi, Indonesia, had revealed genetic adaptations that allowed the Bajau to go for longer periods without oxygen, resulting in unusually large spleens. However, while Jeju residents did, on average, have a larger spleen than the study participants from mainland South Korea, the effect wasn’t significant when other factors like age, height and weight were accounted for, she said. A target for new drugs The genetic variant that the study identified in the Jeju residents associated with lower blood pressure should be explored further, according to Ben Trumble, an associate professor at Arizona State University’s School of Human Evolution and Social Change. “Those with this gene had more than a 10% reduction in blood pressure compared to those who don’t have this gene, that’s a pretty impressive effect,” said Trumble, who wasn’t involved in the study. “Genes code for proteins, and if we can figure out which changes in proteins impact blood pressure, we could potentially create new drugs,” Nearly all medical and genetic studies are conducted in industrialized populations, usually in urban city centers, making Ilardo’s approach particularly valuable, Trumble added. “Almost everything we know about what is ‘normal’ when it comes to health is from these sedentary urban populations. However, for 99.9% of human history, we were hunter-gatherers,” he said. “Natural selection optimized our bodies under very different selective pressures than those we face today.” Ilardo said she hopes to continue to study Jeju’s female divers and get a deeper understanding of the medical implications. “This study raises more questions than it answers, but first and foremost, it shows these women are extraordinary,” she said. “There’s something biologically different about them that makes them extremely special, no matter how you characterize it, and what they do is unique and worth celebrating.”